1. Field of the Invention
The invention pertains generally to electrical switching apparatus and, more particularly, to circuit breakers including a limiter having plural trip indicator members. The invention also relates to limiters and to trip indicators therefor.
2. Background Information
Current limiters employ a current interruption device that rapidly and effectively brings the current to a relatively low or zero value upon the occurrence of a line fault or overload condition.
Circuit protection devices protect electrical equipment from damage when excess current flows in a power circuit due to overload or short circuit conditions. Such devices have a relatively low resistivity and, accordingly, a relatively high conductivity under normal current conditions of the power circuit, but are “tripped” or converted to high or complete resistivity when excessive current and/or temperature occurs. When the device is tripped, a reduced or zero current is allowed to pass in the power circuit, thereby protecting the power circuit conductors and load from electrical and thermal damage until the overload or fault is removed.
Conventional circuit protection or current limiting devices include, but are not limited to, circuit breakers, fuses (e.g., expulsion fuses), thermistors (e.g., PTC (Positive Temperature Coefficient) conductive polymer thermistors), and the like. These devices are current rated for the maximum current the device can carry without interruption under a load.
Circuit breakers typically contain a load sensing element (e.g., a bimetal; a hot-wire; a magnetic element) and separable contacts, which open under overload or short circuit conditions. Most circuit breakers have to be reset manually at the circuit breaker site or through a remote switch.
Fuses typically contain a load sensing fusible element (e.g., metal wire), which when exposed to current (I) of fault magnitude rapidly melts and vaporizes through resistive (R) heating (I2R). Formation of an arc in the fuse, in series with the load, can introduce arc resistance into the power circuit to reduce the peak let-through current to a value significantly lower than the fault current. Expulsion fuses may further contain gas-evolving or arc-quenching materials which rapidly quench the arc upon fusing to eliminate current conduction. Fuses generally are not reusable and must be replaced after overload or short circuit conditions because they are damaged inherently when the power circuit opens.
Various fusible elements, gas-evolving materials and fuses are shown for example in U.S. Pat. Nos. 2,526,448; 3,242,291; 3,582,586; 3,761,660; 3,925,745; 4,008,452; 4,035,755; 4,099,153; 4,166,266; 4,167,723; 4,179,677; 4,251,699; 4,307,368; 4,309,684; 4,319,212; 4,339,742; 4,340,790; 4,444,671; 4,520,337; 4,625,195; 4,638,283; 4,778,958; 4,808,963; 4,950,852; 4,952,900; 4,975,551; 4,995,886; and 5,471,185.
Low voltage circuit breakers are often connected in series with so-called limiters, in order to significantly increase the short circuit switching capacity in low voltage electrical networks and to significantly limit the cut-off currents. Such limiters are designed to transition rapidly, in case of a short circuit, from a low-resistance state to a high-resistance state and, thus, provide rapid current limiting and disconnection. Some limiters employ, for example, fuses, such as fusible wire elements to accomplish this function.
It is known to provide a trip indication for a circuit breaker limiter through a button visible at the top surface thereof. The button changes position responsive to a spring when the button is released by a fused wire element. For example, the button “pops” above the surface of the limiter when the fused wire element burns opens during a fault. See, for example, U.S. Patent Application Publication No. 2006/0267720.
There is room for improvement in electrical switching apparatus and in limiters for such apparatus.
There is also room for improvement in indicators for limiters.